Zoom lens system

ABSTRACT

A zoom lens system includes a negative first lens group, a positive second lens group, and a positive third lens group, in this order from the object. 
     Upon zooming from the short focal length extremity to the long focal length extremity, the distance between the negative first lens group and the positive second lens group decreases, and the distance between the positive second lens group and the positive third lens group increases. 
     The negative first lens group includes a negative first lens element, a negative second lens element having a weaker negative refractive power, and a positive third lens element, in this order from the object. 
     The second lens element of the negative first lens group satisfies the following condition: 
       0&lt;( ra−rb )/( ra+rb )&lt;0.3   (1) 
     wherein 
     ra designates a radius of curvature of the object-side surface of the second lens element of the negative first lens group; and 
     rb designates a radius of curvature of the image-side surface of the second lens element of the negative first lens group.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lens system, which is inexpensive,has adequate telecentricity and a zoom ratio of approximately 4.0, to beused in a small and light-weight digital camera, etc.

2. Description of the Prior Art

In recent years, further miniaturization of digital compact cameras dueto further miniaturization of electronic components have progressed, andfurther miniaturization of the photographing optical system is also indemand.

Furthermore, due to higher pixelization of imaging devices,photographing optical systems are required to have higher resolution.

In order to prevent shading and color shift, excellent telecentricity,by which light emitting from the rearmost lens surface is orthogonallyincident on the imaging surface, is required in the photographingoptical system of a digital camera.

A negative-lead type lens system having a zoom ratio of approximately 3is often utilized as the zoom lens system of a compact digital camera.Since a negative-lead type lens system can achieve a wide angle-of-viewat the short focal length extremity, and can have smaller lensdiameters, especially, the diameter of the most object-side lens group,such a lens system is suitable for a retractable zoom lens system inwhich the distances between lens groups are reduced as the lens groupsare being retracted (in an optical axis direction) to an accommodatingposition.

Furthermore, such a zoom lens system having three lens groups, e.g., alens group having a negative refractive power (hereinafter, a negativelens group), a lens group having a positive refractive power(hereinafter, a positive lens group) and a positive lens group, in thisorder from the object, is often utilized, since the exit pupil has to bepositioned sufficiently away from the imaging plane.

Examples of the prior art can be found in Japanese Unexamined PatentPublication Nos. 2005-70696, 2005-70697, 2005-37727 and 2006-301154.

In the case of a zoom lens system having a zoom ratio exceeding 4.0, itis common to employ a positive-lead type lens system. Such apositive-lead type lens system is suitable for reducing the overalllength of the lens system; however, the diameter of the first lens groupis larger, and is not suitable for the retractable zoom lens camera witha telescopic lens barrel which is arranged to advance and retract.

Japanese Unexamined Patent Publication Nos. 2005-70696, 2005-70697 and2005-37727 disclose relatively small optical systems; however, the zoomratio thereof is approximately 3.0, which is insufficient. Furthermore,cost reduction of the zoom lens systems has not been sufficientlyachieved.

In Japanese Unexamined Patent Publication No. 2006-301154, a zoom lenssystem in which a zoom ratio exceeding 4 is proposed; however, theoverall length of the zoom lens system is long, so that miniaturizationthereof is insufficient. Moreover, a large number of lens elements, andglass aspherical lens elements are employed, which increases the costsof the optical system.

Generally, if attempts are made to achieve an optical system in whichaberrations are suitably corrected without increasing the size of theoptical system, the number of lens elements has to be increased, andmore aspherical lens elements have to be employed, which increases thecosts of the optical system.

In a zoom lens system for a telescopic retractable zoom lens camera, anegative-lead type lens system is desirable. However, if the zoom ratiois increased up to approximately 4.0, the zoom lens system tends to belonger in the optical axis direction, and the correcting of aberrationsbecomes difficult.

SUMMARY OF THE INVENTION

The present invention provides a zoom lens system which is anegative-lead type lens system having three lens groups, has a zoomratio exceeding 4.0, and covers a zooming range (focal length range)from a wide-angle to a telephoto angle; and in the zoom lens system, thecorrecting of aberrations is suitably performed.

The present invention has been devised as a result of researching lensarrangements for negative-lead type lens systems having three lensgroups, especially in regard to the lens arrangement of the first lensgroup.

According to a first aspect of the present invention, there is provideda zoom lens system including a negative first lens group, a positivesecond lens group, and a positive third lens group, in this order fromthe object.

Upon zooming from the short focal length extremity to the long focallength extremity, the distance between the negative first lens group andthe positive second lens group decreases, and the distance between thepositive second lens group and the positive third lens group increases.

The negative first lens group includes a negative first lens element, anegative second lens element having a weaker negative refractive power,and a positive third lens element, in this order from the object.

The second lens element of the negative first lens group satisfies thefollowing condition:

0<(ra−rb)/(ra+rb)<0.3   (1)

wherein

ra designates a radius of curvature of the object-side surface of thesecond lens element of the negative first lens group; and

rb designates a radius of curvature of the image-side surface of thesecond lens element of the negative first lens group.

The zoom lens system satisfies the following condition:

0.3<f1/fp<0.7 (f1<0)   (2)

wherein

f1 designates the focal length of the negative first lens group; and

fp designates the focal length of the second lens element of thenegative first lens group.

The second lens element of the negative first lens group is arranged tohave at least one aspherical surface, and preferably satisfies thefollowing condition:

n2<1.55   (3)

wherein

n2 designates the refractive index of the d-line of the second lenselement.

The most object-side surface of the positive second lens group ispreferably constituted by a concave surface facing toward the image.

According to a second aspect of the present invention, there is provideda zoom lens system including a negative first lens group, a positivesecond lens group, and a positive third lens group, in this order fromthe object.

Upon zooming from the short focal length extremity to the long focallength extremity, the distance between the negative first lens group andthe positive second lens group decreases, and the distance between thepositive second lens group and the positive third lens group increases.

The positive second lens group includes a positive first lens element, anegative second lens element, and a positive or negative third lenselement having a weaker refractive power, in this order from the object.

The third lens element of the positive second lens group preferablysatisfies the following condition:

−0.3<(r3−r4)/(r3+r4)<0.5   (4)

wherein

r3 designates a radius of curvature of the object-side surface of thethird lens element of the positive second lens group; and

r4 designates a radius of curvature of the image-side surface of thethird lens element of the positive second lens group.

In the first and second aspect of the present invention, the zoom lenssystem preferably satisfies the following condition:

0.5<f2/f3<0.9   (5)

wherein

f2 designates the focal length of the positive second lens group, and

f3 designates the focal length of the positive third lens group.

Upon zooming from the short focal length extremity to the long focallength extremity, the positive second lens group is monotonically movedtoward the object, and satisfies the following condition:

3.2<m2t/m2w<4.0   (6)

wherein

m2t designates a magnification of the positive second lens group, at thelong focal length extremity, when an object at infinity is in anin-focus state; and

m2w designates a magnification of the positive second lens group, at theshort focal length extremity, when an object at infinity is in anin-focus state.

Upon zooming from the short focal length extremity to the long focallength extremity, the positive third lens group is monotonically movedtoward the image, and satisfies the following condition:

1.1<m3t/m3w<1.4   (7)

wherein

m3t designates a magnification of the positive third lens group, at thelong focal length extremity, when an object at infinity is in anin-focus state; and

m3w designates a magnification of the positive third lens group, at theshort focal length extremity, when an object at infinity is in anin-focus state.

In the zoom lens system of the present invention, the positive thirdlens group is arranged to function as a focusing lens group, andincludes a biconvex plastic lens element having at least one asphericalsurface.

In the zoom lens system of the present invention, the negative firstlens group preferably satisfies the following condition:

0.2<t1/|f1″<0.4   (8)

wherein

t1 designates a distance from the most object-side surface of thenegative first lens group to the most image-side surface thereof; and

f1 designates the focal length of the negative first lens group.

The present disclosure relates to subject matter contained in JapanesePatent Application No. 2007-202642 (filed on Aug. 3, 2007) which isexpressly incorporated herein in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be discussed below in detail with referenceto the accompanying drawings, in which:

FIG. 1 shows a lens arrangement of a first embodiment of a zoom lenssystem according to the present invention;

FIGS. 2A, 2B, 2C and 2D show aberrations occurred in the lensarrangement shown in FIG. 1 at the short focal length extremity, when anobject at infinity is in an in-focus state;

FIGS. 3A, 3B, 3C and 3D show aberrations occurred in the lensarrangement shown in FIG. 1 at an intermediate focal length, when anobject at infinity is in an in-focus state;

FIGS. 4A, 4B, 4C and 4D show aberrations occurred in the lensarrangement shown in FIG. 1 at the long focal length extremity, when anobject at infinity is in an in-focus state;

FIG. 5 shows a lens arrangement of a second embodiment of a zoom lenssystem according to the present invention;

FIGS. 6A, 6B, 6C and 6D show aberrations occurred in the lensarrangement shown in FIG. 5 at the short focal length extremity, when anobject at infinity is in an in-focus state;

FIGS. 7A, 7B, 7C and 7D show aberrations occurred in the lensarrangement shown in FIG. 5 at an intermediate focal length, when anobject at infinity is in an in-focus state;

FIGS. 8A, 8B, 8C and 8D show aberrations occurred in the lensarrangement shown in FIG. 5 at the long focal length extremity, when anobject at infinity is in an in-focus state;

FIG. 9 shows a lens arrangement of a third embodiment of a zoom lenssystem according to the present invention;

FIGS. 10A, 10B, 10C and 10D show aberrations occurred in the lensarrangement shown in FIG. 9 at the short focal length extremity, when anobject at infinity is in an in-focus state;

FIGS. 11A, 11B, 11C and 11D show aberrations occurred in the lensarrangement shown in FIG. 9 at an intermediate focal length, when anobject at infinity is in an in-focus state;

FIGS. 12A, 12B, 12C and 12D show aberrations occurred in the lensarrangement shown in FIG. 9 at the long focal length extremity, when anobject at infinity is in an in-focus state;

FIG. 13 shows a lens arrangement of a fourth embodiment of a zoom lenssystem according to the present invention;

FIGS. 14A, 14B, 14C and 14D show aberrations occurred in the lensarrangement shown in FIG. 13 at the short focal length extremity, whenan object at infinity is in an in-focus state;

FIGS. 15A, 15B, 15C and 15D show aberrations occurred in the lensarrangement shown in FIG. 13 at an intermediate focal length, when anobject at infinity is in an in-focus state;

FIGS. 16A, 16B, 16C and 16D show aberrations occurred in the lensarrangement shown in FIG. 13 at the long focal length extremity, when anobject at infinity is in an in-focus state;

FIG. 17 shows a lens arrangement of a fifth embodiment of a zoom lenssystem according to the present invention;

FIGS. 18A, 18B, 18C and 18D show aberrations occurred in the lensarrangement shown in FIG. 17 at the short focal length extremity, whenan object at infinity is in an in-focus state;

FIGS. 19A, 19B, 19C and 19D show aberrations occurred in the lensarrangement shown in FIG. 17 at an intermediate focal length, when anobject at infinity is in an in-focus state;

FIGS. 20A, 20B, 20C and 20D show aberrations occurred in the lensarrangement shown in FIG. 17 at the long focal length extremity, when anobject at infinity is in an in-focus state;

FIG. 21 shows a lens arrangement of a sixth embodiment of a zoom lenssystem according to the present invention;

FIGS. 22A, 22B, 22C and 22D show aberrations occurred in the lensarrangement shown in FIG. 21 at the short focal length extremity, whenan object at infinity is in an in-focus state;

FIGS. 23A, 23B, 23C and 23D show aberrations occurred in the lensarrangement shown in FIG. 21 at an intermediate focal length, when anobject at infinity is in an in-focus state;

FIGS. 24A, 24B, 24C and 24D show aberrations occurred in the lensarrangement shown in FIG. 21 at the long focal length extremity, when anobject at infinity is in an in-focus state; and

FIG. 25 is the schematic view of the lens-group moving paths for thezoom lens system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A zoom lens system of the present invention, as shown in the lens-groupmoving paths of FIG. 25, includes a negative first lens group 10, apositive second lens group 20, and a positive third lens group 30, inthis order from the object.

Upon zooming from the short focal length extremity to the long focallength extremity, the distance between the negative first lens group 10and the positive second lens group 20 decreases, and the distancebetween the positive second lens group 20 and the positive third lensgroup 30 increases.

More specifically, the negative first lens group 10 first moves towardthe image, and thereafter moves toward the object, the positive secondlens group 20 monotonically moves toward the object, and the positivethird lens group 30 monotonically moves toward the image.

A diaphragm S is provided between the negative first lens group 10 andthe positive second lens group 20, and integrally moves with thepositive second lens group 20.

Focusing is performed by the positive third lens group 30.

The symbol ‘I’ designates the imaging plane.

As shown in each embodiment of FIGS. 1, 5, 9, 13, 17 and 21, thenegative first lens group 10 includes a negative lens element (firstlens element) 11 having a concave surface facing toward the image, ameniscus lens element (second lens element) 12, with a weaker refractivepower, having the convex surface facing toward the object, and apositive third lens element (third lens element) 13 having a convexsurface facing toward the object, in this order from the object.

The positive second lens group 20 includes a cemented lens having abiconvex positive lens element (fourth lens element: a first lenselement of the positive second lens group 20) 21 and a biconcavenegative lens element (fifth lens element: a second lens element of thepositive second lens group 20) 22, and a positive or negative lenselement 23 having a weaker refractive power (sixth lens element: a thirdlens element of the positive second lens group 20), in this order fromthe object.

The positive third lens group 30 includes a biconvex positive lenselement (seventh lens element) 31.

The symbol ‘C’ designates a cover glass (filter group) positioned infront of an imaging device.

The second lens element 12 of the negative first lens group 10, thesixth lens element 23 of the positive second lens group 20, and theseventh lens element 31 of the positive third lens group 30 are all madeof plastic.

These plastic lens elements can be provided with at least one asphericalsurface.

In the negative first lens group 10, the second lens element 12 of the“weaker refractive power” refers to a lens element in which the paraxialarea does not almost have refractive power, and the peripheral area hasa lens function (aberration-correcting function) by an asphericalsurface. In other words, the second lens element 12 of the “weakerrefractive power” is not designed to have a refractive power at theparaxial area thereof, but to correct aberrations at the peripheralarea.

In the case of a negative-lead type zoom lens system, if an attempt ismade to increase the zoom ratio, the entire zoom lens system becomeslonger. In order to avoid such an increase in length, the refractivepower of each lens group is made stronger so that the traveling distanceof each lens group becomes shorter. However, the amount of aberrationson the refractive surface becomes larger, and the correcting ofaberrations is difficult.

Condition (1) specifies the shape of the second lens element 12 (themeniscus lens element having the convex surface facing toward theobject) in the negative first lens group 10, in the case when thenegative first lens group 10 includes the negative first lens element11, the second lens element 12 with a weaker refractive power, and thepositive third lens element 13, in this order from the object. In otherwords, this condition is to suitably correct aberrations withoutincreasing the thickness of the negative first lens group 10.

If (ra−rb)/(ra+rb) exceeds the lower limit of condition (1), thepositive refractive power of the second lens element 12 becomesstronger. On the other hand, the negative first lens group 10 as a wholehas to have a negative refractive power of predetermined strength, sothat the negative refractive power of the first lens element 11 becomestoo strong to correct coma, astigmatism and lateral chromatic aberrationoccurred in the first lens element 11 at the short focal lengthextremity.

If (ra−rb)/(ra+rb) exceeds the upper limit of condition (2), thenegative refractive power of the second lens element 12 becomesstronger, so that the negative refractive power of the first lenselement 11 becomes relatively weaker. Here, note that the negativerefractive power is distributed over the two negative lens elements 11and 12. This arrangement seems to be advantageous for correctingaberrations; however, the peripheral edge of the second lens element 12becomes thicker, so that the first lens element 11 is inevitably awayfrom the diaphragm S. Consequently, the diameter of the first lenselement 11 becomes larger.

If an attempt is made to forcibly make the diameter of the first lenselement 11 smaller, off-axis aberrations, such as coma, etc., largelyoccur at the short focal length extremity.

Condition (2) specifies the positive refractive power of the second lenselement 12 of the negative first lens group 10. This condition is toattain a shorter focal length at the short focal length extremity and ahigher zoom ratio, while the cost of the zoom lens system is maintainedlower.

If the positive refractive power of the second lens element 12 becomesweaker to the extent that f1/fp exceeds the lower limit of condition(2), it becomes difficult to attain a shorter focal length at the shortfocal length extremity.

If the positive refractive power of the second lens element 12 becomesstronger to the extent that f1/fp exceeds the lower limit of condition(2), a focus-shift due to temperature change undesirably occurs in thecase of plastic lens. In addition, with at least one aspherical surface,the correcting of off-axis aberrations, such as coma and astigmatism,etc., becomes possible when the focal length at the short focal lengthextremity is made shorter.

Condition (3) specifies the refractive index of the second lens element12.

The second lens element 12 of the negative first lens group 10 has aweaker refractive power which satisfies condition (2), so that a glassmaterial which has a low refractive index which satisfies condition (3)can be employed.

For example, in the case where the second lens element 12 is made ofplastic, it becomes easy to form an aspherical surface thereon at lowcost. With at least one aspherical surface, the amount of aberrationsdue to an increase of the zoom ratio can be reduced.

The zoom lens system according to the second aspect of the presentinvention is directed to the positive second lens group 20.

Namely, the positive second lens group 20 includes the three lenselements, i.e., the biconvex positive lens element 21 and the biconcavenegative lens element 22, and the third lens element 23 having a weakerrefractive power; and the third lens element 23 can be provided witheither positive refractive power or negative refractive power.

In the positive second lens group 20, the third lens element 23 of the“weaker refractive power” refers to a lens element in which the paraxialarea does not have refractive power, and the peripheral area has a lensfunction (aberration-correcting function) by an aspherical surface. Inother words, the third lens element 23 of the “weaker refractive power”is not designed to have a refractive power at the paraxial area thereof,but to correct aberrations at the peripheral area.

Condition (4) specifies the shape of the third lens element 23 of thepositive second lens group 20. In other words, this condition is toattain further miniaturization of the zoom lens system and costreduction.

If (r3−r4)/(r3+r4) exceeds the lower limit of condition (4), thecurvature of the image-side surface of the third lens element 23 becomessmaller, so that the effect of diverging off-axis light rays on theimage-side surface is reduced. Consequently, further miniaturization ofthe zoom lens system becomes difficult.

If (r3−r4)/(r3+r4) exceeds the upper limit of condition (4), thenegative refractive power of the third lens element 23 becomes stronger,so that the correcting of coma and astigmatism on the image-side surfaceof the third lens element 23 (i.e., the diverging surface) becomesdifficult.

The third lens element 23 is preferably provided with, on each surface,with an aspherical surface. The aspherical surfaces are arranged tocorrect spherical aberration and coma, over the entire zooming range.The correcting of aberrations can be more effectively made by providingan aspherical surface on each surface of the third lens element 23. Byproviding the two aspherical surfaces, the occurrence of aberrations oneach surface can be reduced. Consequently, it is possible to reduce thesensitivity on deterioration of optical performance due to decentrationof a lens element in an assembly stage.

In the optical system of a digital camera, telecentricity is required;however, such telecentricity prevents the digital camera from attainingfurther miniaturization thereof.

In order to maintain telecentricity and make the entire zoom lens systemshorter, further optical-design consideration is necessary indistribution of refractive power over each lens group, and in lensarrangements. In this respect, the zoom lens system of the presentinvention is preferably arranged to form the final surface (the mostimage-side surface) of the positive second lens group 20 as a divergingsurface.

With the diverging surface formed at the final surface of the positivesecond lens group 20, the height of a bundle of off-axis light rays fromthe optical axis is increased at the short focal length extremity; andthe bundle of off-axis light rays is made parallel with the optical axisby the positive third lens group 30 having a relatively strongerrefractive power. Due to this arrangement, telecentricity is maintainedand the entire zoom lens system is made shorter at the same time, andthe distance between the positive second lens group 20 and the positivethird lens group 30 is made shorter.

In the zoom lens system according to the present invention, the positivethird lens group 30 is arranged to be a focusing lens group. By movingthe positive third lens group 30 along the optical axis, an object at aclose distance is brought into an in-focus state.

The positive third lens group 30 preferably satisfies condition (5).

Condition (5) specifies the refractive power of the positive third lensgroup 30 with respect to that of the positive second lens group 20. Inother words, condition (5) is provided to attain telecentricity, i.e.,diverging off-axis light rays from the final surface of the positivesecond lens group 20 are made parallel with the optical axis by thepositive third lens group 30.

If f2/f3 exceeds the lower limit of condition (5), the refractive powerof the positive third lens group 30 becomes weaker. Consequently, itbecomes difficult to maintain telecentricity, while miniaturization ofthe zoom lens system is attained. The only way to achieve telecentricitywhile maintaining miniaturization is to increase the refractive power ofeach lens group. However, in such a case, the correcting of aberrationsis difficult over the entire zooming range.

If f2/f3 exceeds the upper limit of condition (5), the refractive powerof the positive third lens group 30 becomes too strong with respect tothe positive second lens group 20. As a result, optical performance ofthe zoom lens system upon focusing undesirably varies according to thechange in a closer distance.

In the zoom lens system of the present invention, as shown in thelens-group moving paths of FIG. 21, upon zooming from the short focallength extremity to the long focal length extremity, the positive secondlens group 20 monotonically moves toward the object, and the positivethird lens group 30 monotonically moves toward the image.

Condition (6) specifies the ratio of a magnification of the positivesecond lens group 20 at the short focal length extremity to amagnification thereof at the long focal length extremity.

Condition (7) specifies the ratio of a magnification of the positivethird lens group 30 at the short focal length extremity to amagnification thereof at the long focal length extremity.

These conditions are provided for the purpose of suitably determiningthe refractive power of the positive second lens group 20 and that ofthe positive third lens group 30, and suitably positioning the positivesecond lens group 20 and the positive third lens group 30 along theoptical axis, when the zoom ratio of the zoom lens system is increased.By satisfying these conditions, an increase in size of the zoom lenssystem can be prevented as much as possible.

If m2t/m2w exceeds the lower limit of condition (6), an increase in themagnification of the positive second lens group 20 from the short focallength extremity to the long focal length extremity is smaller, so thatit is difficult to achieve a desired zoom ratio.

If m2t/m2w exceeds the upper limit of condition (6), the magnificationof the positive second lens group 20 becomes too large, so that thetraveling distance of the positive second lens group 20 becomes longer.Consequently, further miniaturization of the zoom lens system becomedifficult. On the other hand, miniaturization of the zoom lens systemwould be possible by making the refractive power stronger (i.e., thetraveling distance thereof is made shorter); however, the correcting ofaberrations becomes difficult over the entire zooming range from theshort focal length extremity to the long focal length extremity; and anoptimal optical performance cannot be achieved.

If m3t/m3w exceeds the lower limit of condition (7), an increase in themagnification of the positive third lens group 30 from the short focallength extremity to the long focal length extremity is smaller.Consequently, it either becomes difficult to achieve a desired zoomratio, or the burden of zooming on the positive second lens group 20becomes larger.

If m3t/m3w exceeds the upper limit of condition (7), the travelingdistance of the positive third lens group 30 becomes longer. Thereforethe positive refractive power of the positive third lens group 30 has tobe made stronger to avoid an insufficient (too-short) back focaldistance at the long focal length extremity. Consequently, it becomesdifficult to reduce changes in field curvature when an object at closedistance is photographed at the long focal length extremity.

The third lens group 30 includes a positive single lens element which ismade of plastic, so that cost reduction can be attained.

Furthermore, by forming an aspherical surface on each surface of thethird lens group 30, deterioration in optical performance when focusingis performed for an object at a closer can be reduced.

In the zoom lens system of the present invention, the negative firstlens group 10 includes three lens elements, so that the negative firstlens group 10 tends to be thicker.

Condition (8) specifies the thickness (length) of the negative firstlens group 10 in order to reduce the length of the zoom lens system atan accommodation position.

If t1/|f1| exceeds the lower limit of Condition (8), the thickness(length) of the negative first lens group 10 becomes too small (short)with respect to the focal length of the negative first lens group 10, sothat the correcting of off-axis aberrations at the short focal lengthextremity becomes difficult.

If t1/|f1| exceeds the upper limit of Condition (8), the thickness(length) of the negative first lens group 10 becomes too large (long)with respect to the focal length of the negative first lens group 10, sothat the length of the zoom lens system at the accommodation positionundesirably becomes longer.

Specific numerical data of the embodiments will be describedhereinafter.

In the diagrams of chromatic aberration (axial chromatic aberration)represented by spherical aberration, the solid line and the two types ofdotted lines respectively indicate spherical aberrations with respect tothe d, g and C lines.

In the diagrams of lateral chromatic aberration, the two types of dottedlines respectively indicate magnification with respect to the g and Clines; however, the d line as the base line coincides with the ordinate.

In the diagrams of astigmatism, y designates the image height, Sdesignates the sagittal image, and M designates the meridional image.

In the tables, FNO. designates the F-number, f designates the focallength of the entire zoom lens system, W designates the halfangle-of-view (°), fB designates the back focal distance, r designatesthe radius of curvature, d designates the lens-element thickness or adistance between lens elements (lens groups), N_(d) designates therefractive index of the d-line, and ν designates the Abbe number.

The values of the F-number, the focal length of the entire zoom lenssystem (f), the half angle-of-view (°) (W) and the back focal distance(fB), and the values of the lens-element thickness or a distance betweenlens elements (lens groups) (d) are indicated in the order of the shortfocal length extremity, an intermediate focal length and the long focallength extremity.

In addition to the above, an aspherical surface which is symmetricalwith respect to the optical axis is defined as follows:

x=cy ²/[1+{1−(1+K)c ²y²}^(1/2) ]+A4y ⁴ +A6y⁶ +A8y ⁸ +A10y ¹⁰

wherein:

-   c designates a curvature of the aspherical vertex (1/r);-   y designates a distance from the optical axis;-   K designates the conic coefficient; and-   A4 designates a fourth-order aspherical coefficient;-   A6 designates a sixth-order aspherical coefficient;-   A8 designates a eighth-order aspherical coefficient; and-   A10 designates a tenth-order aspherical coefficient.

Embodiment 1

FIG. 1 shows the lens arrangement of the first embodiment of a zoom lenssystem according to the present invention. FIGS. 2A through 2D showaberrations occurred in the lens arrangement shown in FIG. 1, when anobject at infinity is in an in-focus state at the short focal lengthextremity. FIGS. 3A through 3D show aberrations occurred in the lensarrangement shown in FIG. 1, when an object at infinity is in anin-focus state at an intermediate focal length. FIGS. 4A through 4D showaberrations occurred in the lens arrangement shown in FIG. 1, when anobject at infinity is in an in-focus state at the long focal lengthextremity.

Table 1 shows the numerical data of the first embodiment.

The negative first lens group 10 (surface Nos. 1 through 6) includes abiconcave negative lens element (first lens element) 11, a negativemeniscus lens element (second lens element) 12 having the convex surfacefacing toward the object, and a positive meniscus lens element (thirdlens element) 13 having the convex surface facing toward the object, inthis order from the object. Note that the negative meniscus lens element(second lens element) 12 is made of plastic, has aspherical surfaces onboth surfaces, and has a weak negative refractive power.

The positive second lens group 20 (surface Nos. 7 through 11) includes acemented lens having a biconvex positive lens element (fourth lenselement) 21 and a biconcave negative lens element (fifth lens element)22, and a positive meniscus lens element (sixth lens element) 23 havingthe convex surface facing toward the object, in this order from theobject. Note that the positive meniscus lens element 23 is made ofplastic, has aspherical surfaces on both surfaces, and has a weakpositive refractive power.

The positive third lens group 30 (surface Nos. 12 through 13) includes abiconvex plastic lens element 31 (seventh lens element) havingaspherical surfaces on both surfaces thereof.

Surface Nos. 14 through 17 define a cover glass (filter group) C whichis positioned in front of an imaging device.

A diaphragm S is provided 0.600 in front (on the object side) of thepositive second lens group 20 (surface No. 7).

TABLE 1 FNo. = 1:2.6-3.6-6.1 f = 5.00-9.16-20.50 (Zoom Ratio = 4.10) W =37.1-22.7-10.4 Surf.No. r d Nd ν  1 −87.039 0.70 1.77250 49.6  2 10.4390.38 — —  3* 10.371 0.80 1.54358 55.7  4* 5.871 1.71 — —  5 10.107 1.761.84666 23.8  6 23.099 17.97-8.91-1.80  — —  7 5.438 3.26 1.69680 55.5 8 −6.854 0.73 1.67270 32.1  9 7.525 0.25 — — 10* 5.110 0.75 1.5435855.7 11* 6.061  2.57-9.33-21.68 — — 12* 22.364 2.00 1.54358 55.7 13*−19.483 4.09-2.60-1.03 — — 14 ∞ 0.50 1.51633 64.1 15 ∞ 0.60 — — 16 ∞0.50 1.51633 64.1 17 ∞ 0.59 (=fb) The symbol * designates the asphericalsurface which is rotationally symmetrical with respect to the opticalaxis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

Surf. K A4 A6 No. 3 −0.10 −0.35049 × 10⁻³     0.20147 × 10⁻⁴ No. 4 −0.10−0.37305 × 10⁻³     0.21658 × 10⁻⁴ No. 10 0.14178 0.77122 × 10⁻³  0.20112 × 10⁻⁴ No. 11 −0.60850 0.78422 × 10⁻² −0.13693 × 10⁻³ No. 12−0.10 0.70570 × 10⁻³ −0.65015 × 10⁻⁴ No. 13 −0.10 0.12841 × 10⁻²−0.11105 × 10⁻³ Surf. A8 A10 No. 3 −0.12798 × 10⁻⁶ No. 4 −0.17746 × 10⁻⁸−0.17233 × 10⁻⁸ No. 10 −0.53935 × 10⁻⁴ No. 11 −0.35721 × 10⁻⁴ No. 12  0.50974 × 10⁻⁵ −0.10361 × 10⁻⁶ No. 13   0.74453 × 10⁻⁵ −0.14500 × 10⁻⁶

Embodiment 2

FIG. 5 shows the lens arrangement of the second embodiment of a zoomlens system according to the present invention. FIGS. 6A through 6D showaberrations occurred in the lens arrangement shown in FIG. 5, when anobject at infinity is in an in-focus state at the short focal lengthextremity. FIGS. 7A through 7D show aberrations occurred in the lensarrangement shown in FIG. 5, when an object at infinity is in anin-focus state at an intermediate focal length. FIGS. 8A through 8D showaberrations occurred in the lens arrangement shown in FIG. 5, when anobject at infinity is in an in-focus state at the long focal lengthextremity.

Table 2 shows the numerical data of the second embodiment.

The basic lens arrangement of the second embodiment is the same as thatof the first embodiment except that the first lens element 11 of thenegative first lens group 10 is a plano-concave lens element (morespecifically, a negative meniscus lens element having the convex surfaceslightly protruding toward the object).

The diaphragm S is provided 0.600 in front (on the object side) of thepositive second lens group 20 (surface No. 7).

TABLE 2 FNo. = 1:2.6-3.7-6.1 f = 5.00-9.15-20.50 (Zoom Ratio = 4.10) W =36.4-22.7-10.3 Surf. No. r d Nd ν  1 1083.377 0.70 1.77250 49.6  2 9.0770.55 — —  3* 9.974 0.80 1.54358 55.7  4* 6.082 1.89 — —  5 10.879 1.921.84666 23.8  6 26.249 18.59-9.17-1.69  — —  7 5.547 2.88 1.77250 49.6 8 −9.756 0.76 1.71736 29.5  9 5.807 0.55 — — 10* 4.672 0.72 1.5091556.4 11* 6.532  2.26-9.51-22.33 — — 12* 16.071 2.00 1.54358 55.7 13*−29.867 4.39-2.80-1.00 — — 14 ∞ 0.50 1.51633 64.1 15 ∞ 0.60 — — 16 ∞0.50 1.51633 64.1 17 ∞ 0.59 (=fb) The symbol * designates the asphericalsurface which is rotationally symmetrical with respect to the opticalaxis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

Surf. K A4 A6 No. 3 −0.10 −0.34425 × 10⁻³     0.21617 × 10⁻⁴ No. 4 −0.10−0.43858 × 10⁻³     0.23008 × 10⁻⁴ No. 10 0.14178 0.10311 × 10⁻²−0.34523 × 10⁻⁴ No. 11 −0.60850 0.74328 × 10⁻² −0.11044 × 10⁻³ No. 12−0.10 0.66938 × 10⁻³ −0.66595 × 10⁻⁴ No. 13 −0.10 0.12248 × 10⁻²−0.12218 × 10⁻³ Surf. A8 A10 No. 3 −0.10929 × 10⁻⁶ No. 4   0.57809 ×10⁻⁸ −0.17938 × 10⁻⁸ No. 10 −0.53668 × 10⁻⁴ No. 11 −0.42133 × 10⁻⁴ No.12   0.46286 × 10⁻⁵ −0.10344 × 10⁻⁶ No. 13   0.79887 × 10⁻⁵ −0.18272 ×10⁻⁶

Embodiment 3

FIG. 9 shows the lens arrangement of the third embodiment of a zoom lenssystem according to the present invention. FIGS. 10A through 10D showaberrations occurred in the lens arrangement shown in FIG. 9, when anobject at infinity is in an in-focus state at the short focal lengthextremity. FIGS. 11A through 11D show aberrations occurred in the lensarrangement shown in FIG. 9, when an object at infinity is in anin-focus state at an intermediate focal length. FIGS. 12A through 12Dshow aberrations occurred in the lens arrangement shown in FIG. 9, whenan object at infinity is in an in-focus state at the long focal lengthextremity.

Table 3 shows the numerical data of the third embodiment.

The basic lens arrangement of the third embodiment is the same as thatof the first embodiment.

The diaphragm S is provided 0.600 in front (on the object side) of thepositive second lens group 20 (surface No. 7).

TABLE 3 FNo. = 1:2.6-3.7-6.2 f = 5.00-9.15-21.00 (Zoom Ratio = 4.20) W =37.1-22.8-10.1_([hasu4-1]) Surf. No. r d Nd ν  1 −96.725 0.70 1.7725049.6  2 11.502 0.07 — —  3* 9.436 0.80 1.54358 55.7  4* 5.354 2.05 — — 5 10.029 1.73 1.84666 23.8  6 21.534 18.67-9.75-1.81 — —  7 5.511 3.031.77250 49.6  8 −8.043 0.70 1.69895 30.0  9 5.744 0.21 — — 10* 4.8690.70 1.54358 55.7 11* 6.149  2.70-9.88-22.43 — — 12* 17.904 2.00 1.5435855.7 13* −20.934 4.02-2.30-1.20 — — 14 ∞ 0.50 1.51633 64.1 15 ∞ 0.60 — —16 ∞ 0.50 1.51633 64.1 17 ∞ 0.59 (=fb) The symbol * designates theaspherical surface which is rotationally symmetrical with respect to theoptical axis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):.

Surf. K A4 A6 No. 3 −0.10 −0.39657 × 10⁻³     0.20176 × 10⁻⁴ No. 4 −0.10−0.35820 × 10⁻³     0.21867 × 10⁻⁴ No. 10 0.14178 0.78351 × 10⁻³−0.13682 × 10⁻⁴ No. 11 −0.60850 0.77284 × 10⁻² −0.12635 × 10⁻³ No. 12−0.10 0.71655 × 10⁻³ −0.66107 × 10⁻⁴ No. 13 −0.10 0.11513 × 10⁻²−0.11029 × 10⁻³ Surf. A8 A10 No. 3 −0.13454 × 10⁻⁶ No. 4 −0.35047 × 10⁻⁹−0.13459 × 10⁻⁸ No. 10 −0.57548 × 10⁻⁴ No. 11 −0.39092 × 10⁻⁴ No. 12  0.49265 × 10⁻⁵ −0.93248 × 10⁻⁷ No. 13   0.77102 × 10⁻⁵ −0.15280 × 10⁻⁶

Embodiment 4

FIG. 13 shows the lens arrangement of the fourth embodiment of a zoomlens system according to the present invention. FIGS. 14A through 14Dshow aberrations occurred in the lens arrangement shown in FIG. 13, whenan object at infinity is in an in-focus state at the short focal lengthextremity. FIGS. 15A through 15D show aberrations occurred in the lensarrangement shown in FIG. 13, when an object at infinity is in anin-focus state at an intermediate focal length. FIGS. 16A through 16Dshow aberrations occurred in the lens arrangement shown in FIG. 13, whenan object at infinity is in an in-focus state at the long focal lengthextremity.

Table 4 shows the numerical data of the fourth embodiment.

The basic lens arrangement of the fourth embodiment is the same as thatof the first embodiment.

The diaphragm S is provided 0.600 in front (on the object side) of thepositive second lens group 20 (surface No. 7).

TABLE 4 FNo. = 1:2.6-3.6-5.8 f = 5.00-9.17-19.60 (Zoom Ratio = 3.92) W =41.8-23.2-11.1 Surf. No. r d Nd ν  1 −89.047 0.70 1.77250 49.6  2 10.4240.39 — —  3* 10.475 0.80 1.54358 55.7  4* 5.915 1.70 — —  5 10.152 1.751.84666 23.8  6 23.164 18.17-9.06-2.16  — —  7 5.437 3.27 1.69680 55.5 8 −6.978 0.74 1.67270 32.1  9 7.486 0.25 — — 10* 5.114 0.75 1.5435855.7 11* 6.057  2.38-9.24-20.75 — — 12* 23.706 2.00 1.54358 55.7 13*−19.512 4.27-2.74-1.40 — — 14 ∞ 0.50 1.51633 64.1 15 ∞ 0.60 — — 16 ∞0.50 1.51633 64.1 17 ∞ 0.59(=fb) The symbol * designates the asphericalsurface which is rotationally symmetrical with respect to the opticalaxis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

Surf. K A4 A6 No. 3 −0.10 −0.34968 × 10⁻³     0.20151 × 10⁻⁴ No. 4 −0.10−0.37431 × 10⁻³     0.21648 × 10⁻⁴ No. 10 0.14178 0.76929 × 10⁻³  0.21448 × 10⁻⁴ No. 11 −0.60850 0.78461 × 10⁻² −0.13837 × 10⁻³ No. 12−0.10 0.71065 × 10⁻³ −0.64522 × 10⁻⁴ No. 13 −0.10 0.12808 × 10⁻²−0.11129 × 10⁻³ Surf. A8 A10 No. 3 −0.12793 × 10⁻⁶ No. 4 −0.19440 × 10⁻⁸−0.17254 × 10⁻⁸ No. 10 −0.54008 × 10⁻⁴ No. 11 −0.35681 × 10⁻⁴ No. 12  0.51118 × 10⁻⁵ −0.10417 × 10⁻⁶ No. 13   0.74253 × 10⁻⁵ −0.14172 × 10⁻⁶

Embodiment 5

FIG. 17 shows the lens arrangement of the fifth embodiment of a zoomlens system according to the present invention. FIGS. 18A through 18Dshow aberrations occurred in the lens arrangement shown in FIG. 17, whenan object at infinity is in an in-focus state at the short focal lengthextremity. FIGS. 19A through 19D show aberrations occurred in the lensarrangement shown in FIG. 17, when an object at infinity is in anin-focus state at an intermediate focal length. FIGS. 20A through 20Dshow aberrations occurred in the lens arrangement shown in FIG. 17, whenan object at infinity is in an in-focus state at the long focal lengthextremity.

Table 5 shows the numerical data of the fifth embodiment.

The basic lens arrangement of the fifth embodiment is the same as thatof the first embodiment except that the third lens element 23 of thepositive second lens group 20 has a weak negative refractive power atthe paraxial area.

The diaphragm S is provided 0.600 in front (on the object side) of thepositive second lens group 20 (surface No. 7).

TABLE 5 FNo. = 1:2.6-3.6-6.0 f = 5.00-9.14-19.60 (Zoom Ratio = 3.92) W =40.8-22.5-10.8 Surf. No. r d Nd ν  1 −98.267 0.70 1.77250 49.6  2 9.0310.75 — —  3* 10.826 0.80 1.54358 55.7  4* 7.142 1.69 — —  5 10.625 1.671.84666 23.8  6 22.900 17.74-8.41-2.16  — —  7 5.654 1.76 1.83481 42.7 8 −20.401 0.75 1.94595 18.0  9 156.652 0.93 — — 10* 7.701 0.78 1.6064127.2 11* 3.420  3.49-9.03-20.59 — — 12* 37.183 2.00 1.54358 55.7 13*−10.959 3.22-2.59-1.40 — — 14 ∞ 0.50 1.51633 64.1 15 ∞ 0.60 — — 16 ∞0.50 1.51633 64.1 17 ∞ 0.59 (=fb) The symbol * designates the asphericalsurface which is rotationally symmetrical with respect to the opticalaxis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

Surf. K A4 A6 No. 3 −0.10 −0.32359 × 10⁻³ 0.20619 × 10⁻⁴ No. 4 −0.10−0.41240 × 10⁻³ 0.20978 × 10⁻⁴ No. 10 0.00 −0.56728 × 10⁻² 0.11017 ×10⁻³ No. 11 0.00 −0.56929 × 10⁻² 0.14784 × 10⁻³ No. 12 −0.10   0.80563 ×10⁻³ −0.64814 × 10⁻⁴   No. 13 −0.10   0.14128 × 10⁻² −0.10305 × 10⁻³  Surf. A8 A10 No. 3 −0.11634 × 10⁻⁶ No. 4 −0.13815 × 10⁻⁷ −0.18478 × 10⁻⁸No. 10   0.50295 × 10⁻⁶ No. 11 −0.19837 × 10⁻⁵ No. 12   0.48467 × 10⁻⁵−0.11101 × 10⁻⁶ No. 13   0.69262 × 10⁻⁵ −0.15829 × 10⁻⁶

Embodiment 6

FIG. 21 shows the lens arrangement of the sixth embodiment of a zoomlens system according to the present invention. FIGS. 22A through 22Dshow aberrations occurred in the lens arrangement shown in FIG. 21 atthe short focal length extremity, when an object at infinity is in anin-focus state. FIGS. 23A through 23D show aberrations occurred in thelens arrangement shown in FIG. 21 at an intermediate focal length, whenan object at infinity is in an in-focus state. FIGS. 24A through 24Dshow aberrations occurred in the lens arrangement shown in FIG. 21 atthe long focal length extremity, when an object at infinity is in anin-focus state.

Table 6 shows the numerical data of the sixth embodiment.

The basic lens arrangement of the sixth embodiment is the same as thatof the fifth embodiment.

The diaphragm S is provided 0.600 in front (on the object side) of thepositive second lens group 20 (surface No. 7).

TABLE 6 FNo. = 1:2.6-3.6-6.1 f = 5.00-9.11-20.50 (Zoom Ratio = 4.10) W =40.9-22.2-10.2 Surf. No. r d Nd ν  1 −41.339 0.70 1.77250 49.6  2 10.8870.20 — —  3* 7.612 0.80 1.54358 55.7  4* 5.387 2.16 — —  5 11.218 1.671.84666 23.8  6 25.236 17.65-8.42-1.73  — —  7 5.519 1.82 1.77250 49.6 8 −15.070 0.95 1.75520 27.5  9 114.131 0.74 — — 10* 7.399 0.81 1.6370023.0 11* 3.502  3.98-9.46-21.59 — — 12* 40.140 2.00 1.54358 55.7 13*−9.752 2.87-2.20-1.15 — — 14 ∞ 0.50 1.51633 64.1 15 ∞ 0.60 — — 16 ∞ 0.501.51633 64.1 17 ∞ 0.59 (=fb) The symbol * designates the asphericalsurface which is rotationally symmetrical with respect to the opticalaxis.

Aspherical surface data (the aspherical surface coefficients notindicated are zero (0.00)):

Surf. K A4 A6 No. 3 −0.10 −0.28038 × 10⁻³ 0.20782 × 10⁻⁴ No. 4 −0.10−0.38683 × 10⁻³ 0.21837 × 10⁻⁴ No. 10 0.00 −0.56599 × 10⁻² 0.96928 ×10⁻⁴ No. 11 0.00 −0.57307 × 10⁻² 0.13246 × 10⁻³ No. 12 −0.10   0.79253 ×10⁻³ −0.63040 × 10⁻⁴   No. 13 −0.10   0.15896 × 10⁻² −0.98458 × 10⁻⁴  Surf. A8 A10 No. 3 −0.11682 × 10⁻⁶ No. 4   0.12938 × 10⁻⁷ −0.16560 ×10⁻⁸ No. 10 −0.35870 × 10⁻⁵ No. 11 −0.41804 × 10⁻⁵ No. 12   0.49521 ×10⁻⁵ −0.10490 × 10⁻⁶ No. 13   0.69112 × 10⁻⁵ −0.15147 × 10⁻⁶

The numerical values of each condition for each embodiment are shown inTable 7.

TABLE 7 Embod. Embod. Embod. Embod. 1 2 3 Embod. 4 Embod. 5 6 Cond.(1)0.277 0.242 0.276 0.278 0.205 0.171 Cond. (2) 0.430 0.409 0.418 0.4280.440 0.441 Cond. (3) 1.544 1.544 1.544 1.544 1.544 1.544 Cond. (4)−0.085 −0.166 −0.116 −0.084 0.385 0.357 Cond. (5) 0.597 0.626 0.6620.584 0.719 0.774 Cond. (6) 3.320 3.226 3.381 3.229 3.344 3.486 Cond.(7) 1.235 1.271 1.243 1.214 1.173 1.176 Cond. (8) 0.359 0.375 0.3430.360 0.382 0.374

As can be understood from Table 7, the first through sixth embodimentssatisfy conditions (1) through (8). Furthermore, as can be understoodfrom the aberration diagrams, the various aberrations are suitablycorrected.

According to the present invention, a negative-lead type zoom lenssystem of three lens groups (i.e., a negative first lens group, apositive second lens group and a positive third lens group, in thisorder from the object) with the following features can be attained:

aberrations are suitably corrected;

a zoom ratio exceeds 4.0; and

a zooming range (focal length range) extends from a wide-angle to atelephoto angle.

Obvious changes may be made in the specific embodiments of the presentinvention described herein, such modifications being within the spiritand scope of the invention claimed. It is indicated that all mattercontained herein is illustrative and does not limit the scope of thepresent invention.

1. A zoom lens system comprises a negative first lens group, a positive second lens group, and a positive third lens group, in this order from an object, wherein upon zooming from the short focal length extremity to the long focal length extremity, the distance between said negative first lens group and said positive second lens group decreases, and the distance between said positive second lens group and said positive third lens group increases; wherein said negative first lens group comprises a negative first lens element, a negative second lens element having a weaker negative refractive power, and a positive third lens element, in this order from the object; and wherein said second lens element of said negative first lens group satisfies the following condition: 0<(ra−rb)/(ra+rb)<0.3 wherein ra designates a radius of curvature of the object-side surface of said second lens element of said negative first lens group; and rb designates a radius of curvature of the image-side surface of said second lens element of said negative first lens group.
 2. The zoom lens system according to claim 1, satisfying following condition: 0.3<f1/fp<0.7 (f1<0) wherein f1 designates the focal length of said negative first lens group; and fp designates the focal length of said second lens element of said negative first lens group.
 3. The zoom lens system according to claim 1, wherein said second lens element of said negative first lens group comprises at least one aspherical surface; and wherein said second lens element satisfies the following condition: n2<1.55 wherein n2 designates the refractive index of the d-line of said second lens element.
 4. The zoom lens system according to claim 1, wherein the most image-side surface of said positive second lens group comprises a concave surface facing toward the image.
 5. The zoom lens system according to claim 1, wherein said positive second lens group comprises a positive first lens element, a negative second lens element, and a positive or negative third lens element having a weaker refractive power, in this order from the object; and wherein said third lens element of said positive second lens group satisfies the following condition: −0.3<(r3−r4)/(r3+r4)<0.5 wherein r3 designates a radius of curvature of the object-side surface of said third lens element of said positive second lens group; and r4 designates a radius of curvature of the image-side surface of said third lens element of said positive second lens group.
 6. The zoom lens system according to claim 1, satisfying following condition: 0.5<f2/f3<0.9 wherein f2 designates the focal length of said positive second lens group, and f3 designates the focal length of said positive third lens group.
 7. The zoom lens system according to claim 1, wherein upon zooming from the short focal length extremity to the long focal length extremity, said positive second lens group is monotonically moved toward the object, and satisfies the following condition: 3.2<m2t/m2w<4.0 wherein m2t designates a magnification of said positive second lens group, at the long focal length extremity, when an object at infinity is in an in-focus state; and m2w designates a magnification of said positive second lens group, at the short focal length extremity, when an object at infinity is in an in-focus state.
 8. The zoom lens system according to claim 1, wherein upon zooming from the short focal length extremity to the long focal length extremity, said positive third lens group is monotonically moved toward the image, and satisfies the following condition: 1.1<m3t/m3w<1.4 wherein m3t designates a magnification of said positive third lens group, at the long focal length extremity, when an object at infinity is in an in-focus state; and m3w designates a magnification of said positive third lens group, at the short focal length extremity, when an object at infinity is in an in-focus state.
 9. The zoom lens system according to claim 1, wherein said positive third lens group is arranged to function as a focusing lens group, and comprises a biconvex plastic lens element having at least one aspherical surface.
 10. The zoom lens system according to claim 1, satisfying the following condition: 0.2<t1/|f1|<0.4 wherein t1 designates a distance from the most object-side surface of said negative first lens group to the most image-side surface thereof; and f1 designates the focal length of said negative first lens group.
 11. A zoom lens system comprises a negative first lens group, a positive second lens group, and a positive third lens group, in this order from an object, wherein upon zooming from the short focal length extremity to the long focal length extremity, the distance between said negative first lens group and said positive second lens group decreases, and the distance between said positive second lens group and said positive third lens group increases; wherein said positive second lens group comprises a positive first lens element, a negative second lens element, and a positive or negative third lens element having a weaker refractive power, in this order from the object; and wherein said third lens element of said positive second lens group satisfies the following condition: −0.3<(r3−r4)/(r3+r4)<0.5 wherein r3 designates a radius of curvature of the object-side surface of said third lens element of said positive second lens group; and r3 designates a radius of curvature of the image-side surface of said third lens element of said positive second lens group.
 12. The zoom lens system according to claim 11, satisfying following condition: 0.5<f2/f3<0.9 wherein f2 designates the focal length of said positive second lens group, and f3 designates the focal length of said positive third lens group.
 13. The zoom lens system according to claim 11, wherein upon zooming from the short focal length extremity to the long focal length extremity, said positive second lens group is monotonically moved toward the object, and satisfies the following condition: 3.2<m2t/m2w<4.0 wherein m2t designates a magnification of said positive second lens group, at the long focal length extremity, when an object at infinity is in an in-focus state; and m2w designates a magnification of said positive second lens group, at the short focal length extremity, when an object at infinity is in an in-focus state.
 14. The zoom lens system according to claim 11, wherein upon zooming from the short focal length extremity to the long focal length extremity, said positive third lens group is monotonically moved toward the image, and satisfies the following condition: 1.1<m3t/m3w<1.4 wherein m3t designates a magnification of said positive third lens group, at the long focal length extremity, when an object at infinity is in an in-focus state; and m3w designates a magnification of said positive third lens group, at the short focal length extremity, when an object at infinity is in an in-focus state.
 15. The zoom lens system according to claim 11, wherein said positive third lens group is arranged to function as a focusing lens group, and comprises a biconvex plastic lens element having at least one aspherical surface.
 16. The zoom lens system according to claim 11, satisfying the following condition: 0.2<t1/|f1|<0.4 wherein t1 designates a distance from the most object-side surface of said negative first lens group to the most image-side surface thereof; and f1 designates the focal length of said negative first lens group. 